3.4.2 Thin Film Structure

This will be short, because we will discuss this topic mostly when we come to deposition methods.

Let's distinguish to separate points

Geometry and morphology.
Internal structure.

The first topic has been mentioned before, but now we go beyond the points made there.

Let's consider to grow a thin film on a substrate that has a small hole in it - say half a µm in lateral size and 1 µm deep. We deposit a thin film (atomic) layer by layer (symbolized by various shade of yellow). What we might get could have one of the topologies shown below or anything in between

The only difference between these quite different topologies is the deposition method; we will cover that in more detail in chapter 6.

There is much more along this line, but we cover it when we get to it.

As far as the internal structure goes, the situation is similar. Everything known from bulk materials goes:

Poly-crystalline thin films with grain sizes ranging from a few nanometers to cm (Are the Zn-covered steel lamp posts, letter boxes, etc. with huge grains products of the thin film industry?)

Single crystalline thin films, but full of defects like dislocations, precipitates, point defects.

Nearly perfect single crystalline thin films - what we often would like to have, but not always get.

If we just look at polycrystalline thin films, we may have just regular grains, or all kinds of textures. Again, we deal with it when we run across it.

Then we have some thin film specialities:

Amorphous thin films, like amorphous Si (a-Si) or many other materials. You just can't have amorphous bulk Si or most everything else that usually likes to form a crystal.

Mixtures of amorphous and crystalline phases; truly nanocrystalline structures (i.e. grain size around 10 nm) - practically never found in bulk.

A case in point is Silicon (what else?). We have:

Amorphous Silicon, used, e.g., in microelectronics. If it is heavily mixed (= "doped" with Hydrogen (> 15 %), we have the crucial thin film for a-Si:H solar cells or for the transistor matrix of liquid crystal displays (LCD). Another example is amorphous SiO₂, the work-horse of microelectronics.

Amorphous-crystalline mixes, like a-Si:H containing nanometer-sized embedded islands of crystalline silicon (c-Si and then called µc-Si:H. This is the base of the so-called "microcrystalline Si thin-film solar cell", one of the hottest contender for the solar cell market of the future.